17 August 2001

Event offers blackleg and phosphate spud pointers

By Andrew Swallow

REASONS for rogueing seed crops for blackleg and continuing concern about DEFRAs RB209 fertiliser advice for potatoes were key issues at the SCRI/SAC Potatoes in Practice event, sponsored by the British Potato Council, near Dundee last week.

SAC disease specialist Stuart Wale said there was little or no relationship between blackleg seen on the stems of the crop and the loading on the seed produced.

"Why are we rogueing and why do we have tolerances for blackleg in the crop if it bears no relation to seed loading? Rogueing is time consuming and costly."

At present, seed growers went through the crop before inspections removing infected plants. But within weeks infection could re-emerge and when canopies had closed, the process of walking through a crop could spread more disease than the rogueing removed, he added. Meanwhile, SAC potato specialist Barry Mitchell suggested growers could be spending much more on phosphate fertilisers for potatoes than was necessary.

Phosphate rates on soils of index 3 or over, or high on the SAC scale (see table) could be cut to a 50kg/ha insurance dressing. "That is equivalent to the off-take of a 50t/ha crop."

Phosphate could even be reduced to zero if recent results in BPC-funded work proved repeatable, he said. "We are not seeing a response in these trials to phosphate applications. At face value, they suggest that if we withhold the phosphate there would be no detrimental effect on tuber number or yield."

Recommendations in the new RB209 book are also thrown into question by the results. It recommends 130kg/ha of phosphate across all crop types at index 3. That was higher than SACs recommendation for maincrop, which itself could be too high, said Mr Mitchell.

RB209 also fails to differentiate between different crop types whereas SAC recommendations are split into five categories.

But Mr Mitchell acknowledged that where phosphate applications on potatoes were aimed at delivering the nutrient needed for the whole rotation then rates as high as 150kg/ha may be justified.

"But an awful lot of potatoes are grown on rented land and you do not want to be leaving behind 100kg of phosphate, unless you are getting a rent reduction." &#42

Blackleg loading

Seed infection Crop infection (% plants)

(bacteria/seed 18/7 7/8


Control* 0.1 1.3

10 1.6 24.7

1,000 16.3 48.8

*Seed stock, assumed infection free.

RB209 v SAC phosphate

Index RB209 SAC (maincrop)

0 270 240

1 230 200

2 180 150

3 130 100

4 50 100

5 0 100